Rebound hardness tests have been used to measure the hardness of specimen, particularly metal material. In the rebound hardness test, generally, an impactor which is constructed by an indenter made of a hard material such as diamond or the like and an indenter support member to which the indenter is secured, is impacted against the surface of the specimen, and a rebound height or a rebound speed of the impactor that has been rebounded from the specimen is measured to thereby measure the hardness of the specimen. Such rebound hardness tests include the Shore hardness test and the Leeb hardness test.
The Shore hardness test, which is specified as a rebound hardness test in JIS (Japanese Industrial Standards), is a hardness testing method in which a hammer serving as the impactor freely drops from a predetermined height (drop height) onto a specimen, and the rebound height representing a maximum point reached by the hammer that has been rebounded from the specimen is measured (see JIS B 7727:2000 “Shore hardness test—Verification of testing machines”). In the Shore hardness test, the hammer is the impactor and includes an indenter and an indenter support member to which the indenter is secured. The Shore hardness is obtained by multiplying a ratio of the rebound height to the drop height of the hammer by a predetermined proportionality constant.
The Leeb hardness test is a hardness testing method in which an impact body serving as the impactor is ejected toward a specimen by a spring, and an impact speed of the impact body before impacting against the specimen and a rebound speed of the impact body when impacting against the specimen and rebounded therefrom (i.e., the speed of the impact body after impacting against the specimen) are measured (see U.S. Pat. No. 4,034,603). In the Leeb hardness test, the impact body is an impactor and includes an indenter and an indenter support member to which the indenter is secured. In the Leeb hardness test, a ratio of the rebound speed of the impact body to the impact speed of the impact body serving as the impactor is measured as a coefficient of restitution. The Leeb hardness is obtained by multiplying the coefficient of restitution by a predetermined proportionality constant.
The rebound hardness tests, which are typified by the Shore hardness test and the Leeb hardness test, are advantageous in that testing of the hardness can be simply and quickly performed, compared with indentation hardness tests such as the Rockwell hardness test and the Vickers hardness test. Furthermore, testers for use in the rebound hardness tests are advantageous in that they have a simple structure and excellent portability, compared with testers for use in the indentation hardness tests.
However, a mass of the hammer for use in the Shore hardness test, and a mass of the impact body for use in the Leeb hardness test are comparatively large. For example, the mass of the hammer of a D-type Shore hardness tester is 36.2 g and the mass of the impact body of the Leeb hardness tester is 5.45 g. When the hardness of a small and light specimen is measured using such hammer or impact body, only a hardness value lower than the true hardness value of this specimen may be obtained.
The reason of this is that the kinetic energy of the impactor (i.e., the hammer or the impact body including the indenter and the indenter support member) which impacts against the small and light specimen is consumed by not only plastic deformation and elastic deformation of the specimen, but also vibrations or the like, of the specimen, resulting in the measurement of a rebound height or a rebound speed which is smaller than the rebound height or the rebound speed to be measured normally. This phenomenon, i.e., the phenomenon that a smaller rebound height or rebound speed than the rebound height or rebound speed to be measured normally, is measured as a result of the consumption of the kinetic energy of the impactor by vibration or the like, of the specimen, will be referred to as “mass effect” in the present specification.
If the mass effect occurs when the hardness of the specimen is measured, a correct hardness of the specimen cannot be obtained. Therefore, when the hardness of a specimen having a mass of 4 kg or less is to be measured by the Shore hardness test, it is necessary to perform the test while the specimen is firmly secured to a dedicated steel anvil having a sufficiently large mass. In the Leeb hardness test, a dedicated anvil is not prepared. Thus, when the hardness of the small and light specimen is to be measured by the Leeb hardness test, it is necessary for the user to prepare an appropriate support having a sufficiently large mass, and firmly secure the specimen to the support using a dedicated paste. Specifically, there have been restrictions on the size and mass of the specimen to be tested when its hardness is to be correctly measured by conventional rebound hardness testers.
In the Shore hardness test, since it is necessary to measure the rebound height after the hammer has dropped freely from a predetermined drop height to impact with the specimen, the testing direction of the Shore hardness test is limited to a vertical direction. On the other hand, in the Leeb hardness test, the impact body is ejected toward the specimen by the spring, and the coefficient of restitution which represents the ratio of the rebound speed of the impact body to the impact speed thereof before the impact body impacts against the specimen is measured. Accordingly, the Leeb hardness test is capable of measuring, in a free direction, the coefficient of restitution and the hardness based on the coefficient of restitution.
According to the hardness testing method, such as the Leeb hardness test, in which an impactor is ejected by the spring to measure the coefficient of restitution of the specimen and this coefficient of restitution is used as an index for evaluating the hardness of the specimen, the hardness test can be performed while the tester is oriented in a free direction. However, when the specimen to measure the hardness is small and light, the mass effect occurs even in the Leeb hardness test. Therefore, there have been demands for an apparatus for measuring coefficient of restitution and a hardness tester which are capable of reducing the mass effect and performing tests in free directions.